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A new method is used to measure the self-diffusion coefficient in gaseous argon as a function of temperature. Data obtained from 77.5°K to 121°K and at room temperature (294°K) agree well with earlier data obtained by Winn with a different method. The data are shown to be consistent with both the viscosity and second virial coefficient of argon. They are also used to estimate the effective long-range dispersion interaction- energy between argon atoms. The value thus derived for the interaction-energy agrees well with sum-rule calculations.

We have used the Dagens-Rasolt-Taylor pseudopotential to carry out a molecular dynamics simulation of S(Q, ω) of solid b.c.c. potassium at temperatures corresponding roughly to 0.5 and 0.9 of melting. The calculated temperature broadening of the one-phonon peaks, and the interference of these peaks with the multiphonon background agree well with experimental neutron scattering data.

Measurements of the isotopic separation for the physical adsorption of argon atoms on graphitized carbon blacks P33 (2700°C) and Spheron 6 have been obtained in the temperature range 78°K to 100°K at several values of the adsorption pressure. Using the grand partition function we have derived a general expression for the separation factor α in terms of the surface-adatom forces and the adatom-adatom forces. Our experimental results are compared with the theory by introducing specific simple models. Values of lnα extrapolated to p → O obey a 1/T2 law and are used with data from low pressure adsorption isotherms to derive the parameters of an assumed surface-adatom potential. The observed pressure dependence of 1nα appears to be inconsistent with the assumption of a planar and energetically homogeneous surface.

Recent experiments on the temperature dependence of the velocity of sound in solid argon are compared with model calculation using a Mie-Lennard-Jones interatomic potential. At high temperatures theory and experiment are in fair agreement, but at low temperatures, the work of Gsanger et al. is in definite disagreement with theory.

Thermodynamic properties of solid Ne are evaluated using an improved self-consistent phonon theory that explicitly includes odd derivatives of the potential. The T dependence of thermodynamic properties of Ne are in good agreement with experiment (except for the isothermal compressibility near the melting point — a difficult quantity to measure!) and differ substantially from the predictions of the lowest order theory.

Contrary to a recent letter, we point out that both theory and experiment support the equality of thermal and elastic Debye temperatures for copper at the absolute zero.